1. Field of the Invention
This invention relates to a method for fabricating an extrusion die and in particular a helical gear extrusion die in which the surface of the die beyond the surface that first forms the contour of the extrusion tapers radially outwardly from the throat or land of the forming surface by reason of the die surface having been formed in a prestressed, elastically deformed condition, which is later removed and the die permitted to assume an equilibrium shape.
2. Description of the Prior Art
In the process of forming articles by extruding metal through a die having the contour of the part to be formed, it is known in the art that the extrusion expands radially outwardly after being forced through the forming surface of the die. The radial expansion is a result of elastic deformations produced in the extrusion blank during the forming process when the blank is forced through the forming throat. The strain energy stored in the part is released after the extrusion blank passes the forming surface of the die.
Unless the die provides a suitably larger space beyond the forming throat or land than the space at the forming surface, the extrusion will expand into abutting engagement with the die surface and produce an excessively high frictional force. This force may in fact exceed the capacity of the metal forming press to force the extrusion blank through the die and could promote galling of the die surface and lead to premature failure. In any case, an excessive amount of energy is required to produce the extrusion due to the large frictional forces that would result.
The prior art has recognized the difficulties associated with this expansion of the extrusion within the die. Extrusion dies typically are formed so as to produce a greater annular space beyond the forming surface than the space defined by the forming surface. Electrical discharge machining EDM methods, wherein one carbon electrode having the desired shape of the part to be formed is passed through the die to establish the die land, are frequently employed. An electrical current produces a spark between the electrode and the inner surface of the die blank, which operates to remove metal from the die surface in the shape of the carbon electrode. In this way the forming surface of the die is established. However, in order to furnish the relief beyond the forming land, the prior art has typically used a second, in this case, tapered electrode to form a cavity beyond the die land which is tapered outwardly to provide the requisite increased volume to accommodate the radial expansion.
However, because two distinct electrodes are used in conventional forming processes, it is extremely difficult to adapt the electrodes to eliminate geometric discontinuities particularly at the cross section where the tapered surface and the land forming surface intersect.
When the extruded part has a noncircular cross section, particularly when it is lobed, as, for example, a gear, an additional difficulty is presented when the inner surface of the die is formed by a process that requires an electrode or forming tool to be inserted into the die for shaping purposes, but then to be withdrawn and the rest of the die surface formed either by another tool or by the same tool introduced into the die from the opposite direction. When this procedure occurs, in addition to the difficulties associated with aligning the centerlines of the respective tools so that the axes are colinear, the tools must be additionally controlled so that at the cross section where the first shaping operation terminates and the second begins, the tools correspond circumferentially. This circumferential registry is required because of the nonuniform shape of the outer contour of the parts. For example, where the die is to be used for extruding gear teeth, the flanks of the teeth on the die must form a continuous and smooth surface along the full length of the die, even though the surface is shaped by two distinct passes of a tool through the die blank. When the gear teeth are helical, the difficulty of maintaining circumferential registry is compounded by the requirement that the forming tools rotate while they pass axially through the die blank.
Other and more conventional methods are known in the art for forming the interior surface of an extrusion die. For example, grinding and, before the die blank is hardened, milling and broaching techniques are available. Each of these, however, presents the same difficulties of circumferential misalignment and dimensional discontinuity as discussed previously with respect to electrical discharge machining. Grinding, however, increases the cost to produce the die.